This invention relates to improvements for a separation process which utilizes a simulated countercurrent flow system wherein a fluid stream flows through serially and circularly interconnected desorption, rectification and sorption zones. More particularly, the improvements concern the use of a dual desorbent composition technique carried out by means of the introduction of two different desorbent streams which minimizes the total amount of desorbent requirements. Additionally in another embodiment, the improved process includes a temperature gradient technique to increase the strength of the desorbent stream employed whereby the desorbent stream is first heated prior to its introduction to the desorption zone with a heat exchanger thereby increasing its desorbent strength and reducing overall desorbent requirements.
The present process employing the above embodiments is an improvement of the simulated countercurrent flow processes described in U.S. Pat. No. 3,761,533 and U.S. Pat. No. 3,201,491. It is known that adsorption-separation processes of liquid feed mixtures, the technique of employing a moving bed type adsorption process wherein said moving bed comprises sorbent particles which are countercurrently contacted with streams of liquid feedstock and desorbent, results in a high degree of purity for the adsorbed product. This process and the so-called simulated countercurrent flow system wherein the solid sorbent particles are stationary have been proposed and disclosed in the above-referred to patents.
In the latter known process, an adsorption separation column is divided into three (or four equivalent) zones: a sorption zone, (a primary rectification zone), a desorption zone and a (secondary) rectification zone. A downstream portion of the sorption zone is also called a primary rectification zone. These zones are serially interconnected in order and a continuously circulated fluid stream flowing through the three (or four) zones is maintained by circulating the effluent fluid from an outlet of the last zone to an inlet of the first zone; all the points of introducing and withdrawing the inlet and outlet streams are simultaneously shifted, at stated intervals of time, in a downstream direction to provide thereby a simulated countercurrent flow system wherein there is achieved a processing effect similar to that observed in the moving-bed type adsorption process. This process for a simulated countercurrent flow system in an adsorption-separation process may be described as follows. In such a process, at least one of the components of the liquid feed mixture is selectively sorbed by contact with solid sorbent particles; said liquid feed mixture is allowed to flow through three serially and circularly interconnected zones: a desorption zone, a rectification zone and a sorption zone, each zone being divided into a plurality of serially interconnected sections, each section being packed with a mass of solid sorbent particles; introducing a desorbent stream into the first section of the desorption zone; introducing the liquid feed mixture to the first section of the sorption zone and withdrawing a raffinate effluent comprising a less sorbed component and the desorbent from the sorption zone; and all the points of introducing and withdrawing the liquid steams into and from the sections are simultaneously shifted, at stated intervals of time, in a downstream direction, while maintaining the same order of continuity and the same spatial relationship between all the points.
In conducting the above-described process, several attempts were made to reduce the total desorbent requirements and also enhance the purity of the recovered sorbate. Stine et al., for example, disclosed a process in U.S. Pat. No. 3,201,491 (1965) which employs a portion of the desorption effluent withdrawn from the last section of the desorption zone by passing it directly into the (secondary) rectification zone in order to physically wash the raffinate materials remaining in the inactive void interstices between the active sorbent particles. An externally-prepared purging fluid comprising the sorbate and raffinate components of the feedstock was also claimed in their patent. Another improvement described in U.S. Pat. No. 3,455,815 (Fickel; 1967) envisions the employment of a stream consisting essentially of an inert material in order to flush non-selectively sorbable components of the feedstock from the interstitial void spaces between the sorbent particles in the rectification zone. A third method which is described in U.S. Pat. No. 3,761,533 (Otani et al.; 1973) introduces a portion of the desorption effluent which is rich in sorbate content into the rectification zone for the purpose of enhancing the purity of the sorbate component adsorbed within the rectification zone.
The above methods, however, contain certain deficiencies. The use of a portion of the desorption effluent will not only result in an increase in the desorbent consumption but also can desorb certain amount of sorbate adsorbed within the rectification zone and thereby limit the overall efficiency of the system. Further, Fickel's concept of employing a flushing stream consisting essentially of an inert material may not provide an adequate means to desorb chemically-adsorbed raffinate materials; and, consequently, the purity of the sorbate product may not be satisfactory. It has now been discovered that the employment of the dual desorbent composition technique and/or the dual temperature technique described herein can substantially eliminate the above deficiencies and markedly improve the overall performance of sorption-separation processes.